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SEPARATIONS Role of H2O in Oxidative Regeneration of ZnS Formed from High-Temperature Desulfurization ZnO Sorbent Eiji Sasaoka,* Makoto Hatori, and Norimasa Sada Faculty of Environmental Science and Technology, Okayama University, Tsushima-naka 3-1-1, Okayama 700-8530, Japan
Md. Azhar Uddin Faculty of Engineering, Okayama University, Tsushima-naka 3-1-1, Okayama 700-8530, Japan
To develop an easily regenerable zinc oxide high-temperature desulfurization sorbent, the role of H2O in the course of oxidative regeneration of zinc sulfide in the presence of O2 were studied using ZnO without support material as a sorbent. Oxidative regeneration of ZnS in the presence of water vapor was examined using TPR technique and H218O. From this study, the following results were obtained: ZnS was converted to SO2 and H2 by H2O in the absence of O2 at high temperature (> ca. 600 °C); in the first step in the oxidative regeneration, ZnS predominantly reacted with H2O more than O2; the SO2 formed from ZnS contained oxygen from H2O. From the experimental results, it was found that the oxidative regeneration of ZnS in the presence of H2O and O2 can be expressed by the following two equations: ZnS + 3H2O S ZnO + SO2 + 3H2; 3H2 + 3/2O2 w 3H2O. In the absence of H2O or in the case of very low value of H2O/O2 concentration ratio, the following reaction also occurred: ZnS + 3/2O2 w ZnO + SO2 Introduction Solid-oxide fuel cells and molten-carbonate fuel cells, new technologies using coal-derived gas, are receiving attention from thermal efficiency and/or environmental points of view. To establish these highly efficient processes, it is necessary to develop a high-temperature process for the desulfurization of coal-derived fuel gas.1 Zinc oxide, as a result of its favorable sulfidation thermodynamics, appears to be the most attractive of the sorbents for highly efficient sulfur removal,2 and current practical research seem to be concentrated on zinc titanate and zinc ferrite.3-15 We studied the characteristics of ZnO as a hightemperature desulfurization sorbent: the stability of ZnO under high-temperature desulfurization conditions;16 the nature of the reaction between ZnO and H2S;17 the nature of the reaction between ZnO and COS;18 the catalytic activity of ZnS formed from desulfurization sorbent for conversion of COS to H2S;19 and soot formation over zinc ferrite sorbent.20 From these studies, the reactions that occur in desulfurization conditions have been clarified somewhat. However, to develop highly useful desulfurization sorbents, much more study on the durability for cyclic use of sorbent and/or the regenerative characteristics of the sorbents are necessary. Therefore, we studied the modification of the reactivity and regenerative characteristics of ZnO-TiO2 and found that the presence of H2O accelerated the oxidative regeneration of some sorbent. However, the mechanisms of the acceleration are unknown.21 * E-mail:
[email protected]. Phone and Fax: +81-86-251-8900.
This work focuses on the clarification of the role of H2O on the oxidative regeneration of the ZnS formed from the high-temperature desulfurization sorbent ZnO. Already, the oxidative regeneration of ZnS in the presence of H2O has been reported.3-5 In these papers, it was thought that ZnS directly reacted with O2. However, the role of H2O was not clarified. Experimental Section Preparation of Zinc Oxide Sorbent. Zinc oxide was prepared by a precipitation method using Zn(NO3)2 and NH3. A aqueous raw salt solution (containing 10 wt % of metal salt) and a 2.5 wt % aqueous NH3 solution (containing a 10% excess of the theoretical amount of NH3 required for precipitation) were prepared. The precipitation was carried out by adding the raw salt solution to the NH3 aqueous solution under vigorous mixing at room temperature. Product of the precipitation was washed, separated by filtration, dried at 110 °C for 25 h, and then calcined in an air stream (300 cm3/ min at STP) from room temperature to 600 °C (10 °C/ min) and held for 3 h. The product thus obtained was crushed and sieved to 0.5 mm. The particle size of the sorbent was the same as that of sorbents used in a fluidized-bed reactor. BET surface area and bulk density of the samples were 5.2 m2/g and 0.6 g/cm3, respectively. Sulfided Sample Preparation. The sulfidation of the sample ZnO was carried out using a flow-type packed-bed tubular reactor system under atmospheric pressure at 450 °C. The microreactor consisted of a quartz tube of 0.5 cm i.d., in which 0.2 g of sorbent was packed. A mixture of H2S (500 ppm), H2 (30%), H2O
10.1021/ie0004747 CCC: $19.00 © 2000 American Chemical Society Published on Web 09/14/2000
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Figure 1. S16O2 (Mz 64) evolution during TPR in the absence of oxygen. Sample: 69% sulfided ZnO. Inlet gas: 10% H216O-N2.
(9.8%), and N2 was fed into the reactor at 200 cm3STP/ min. From the sulfidation treatments for 3 and 10 h, 32% and 69% sulfided ZnO samples were obtained, respectively. The sulfided sample particles were carefully mixed, and then a part of the sample was used for characterization. From XRD analysis of the 32% sulfided ZnO, it was confirmed that the ZnS sample contained wurtzite and sphalerite type ZnS. Characterization of Oxidative Regeneration of ZnS. The characteristics of the regeneration of the ZnS samples were examined using TPR (temperature programmed reaction) apparatus equipped with a quadrupole mass spectrometer (Q.Mass). In the TPR examination, a mixture of O2 (3% or 10%), H216O or H218O (0-10%), and He (or N2) was fed into a microreactor in TPR apparatus at 25 cm3STP/min. H218O was obtained from Nippon Sanso Co. Ltd., and its purity was 94.5 atom %. The microreactor was a quartz U tube in which 10 mg of the sample was packed. The sample was heated to 900 °C at a constant rate of 10 °C/min.
Figure 2. Effect of the presence of H218O on sulfur dioxide evolution during TPR. Sample: 69% sulfided ZnO. Inlet gas: 10% 16O -10% H 16O or (0%, 2%, 10%) H 18O-He. 2 2 2
Results and Discussion Reactivity of ZnS with H2O. To confirm the possibility of the reaction between ZnS and H216O, S16O2 evolution during TPR of the ZnS (69% sulfided sample) was measured under the condition of a 10% H216O -N2 system. As shown in Figure 1, desorption of S16O2 (Mz 64) from ZnS was observed above ca. 450 °C. Furthermore, in another TPR experiment using the packed bed reactor, the formation of S16O2 and H2 were detected using GC (not quantitative). From these results, it was found that ZnS directly reacted with H216O and formed S16O2 and H2 at the high temperatures. The reaction may be expressed in the following equation: 16
16
ZnS + 3H2 O S ZnO + S O2 + 3H2
(1)
Evolution of Sulfur Dioxides during TPR of the ZnS in the Presence of H218O. Figure 2 shows the evolution of sulfur dioxides during TPR of the 69% sulfided samples obtained in the presence and absence of H218O. In both the absence and presence of H 216O but in the presence of 10% 16O2, only S16O2 (Mz 64) was monitored. It was confirmed that the presence of H216O accelerated the oxidative regeneration of the ZnS. In the presence of H218O and 10% 16O2, the formation of S16O18O (Mz 66) and S18O2 (Mz 68) was observed. The relative peak intensity of the S18O2 (Mz 68) increased
Figure 3. Effect of the sulfidation degree of ZnS on sulfur dioxide evolution during TPR. Sample: 32% and 69% sulfided ZnO. Inlet gas: 10% 16O2-10% H218O-He.
with increase in of concentration of H218O. At 600 °C, the order of the formation rates of sulfur dioxides in the presence of 10% H218O was as follows:
S18O2 > S16O18O > S16O2
(2)
From comparison of the TPR profile in the presence of H216O and that in the presence of H218O, it wasconfirmed that the difference between the reactivity of H16O2 and that of H18O2 for this reaction was negligible (total profile of S18O2, S16O18O, and S16O2 were not shown). Figure 3 shows the sulfur dioxide evolution during TPR of the 32% sulfided ZnO in comparison with that of the 69% sulfided sample. The ratio of the evolution of S18O2 (Mz 68) to that of S16O2 (Mz 64) during the TPR of the 32% sulfided sample was larger than that of the 69% sulfided sample. This result suggests that the sulfur located near the surface of the porous ZnS more easily captured the oxygen of the H218O. Furthermore, it may be expected that the oxidative regeneration was
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Figure 4. Effect of the concentration of 16O2 on the evolution of sulfur dioxide during TPR. Sample: 69% sulfided ZnO. Inlet gas: 10% 16O2-10% H218O-He.
Figure 5. Effect of the presence of a small amount of H218O on the evolution of sulfur dioxide during TPR. Sample: 69% sulfided ZnO. Inlet gas: 3% 16O2-(0% or 0.4%) H218O-He.
initiated by the reaction of eq 1 and accelerated by the gas-phase oxygen 16O2. The reaction may be expressed as follows:
2H2 + 16O2 w 2H216O
(3)
Equation 3 is exothermic in nature, and the acceleration results from more heat liberation. The formation of S16O18O during the TPR could be explained by the formation of 2H216O, because both the formed H216O and the fed H218O can contribute to S16O18O formation. Furthermore, the formed H216O can contribute to S16O2 formation. Effects of Concentration of 16O2 on the Evolution of Sulfur Dioxides during TPR. To understand the role of 16O2, the effect of the concentration of 16O2 on the evolution of sulfur dioxide was studied in the presence of 2% H218O. As shown in Figure 4, all of the profiles of evolution of sulfur dioxide were shifted by increase in the concentration of 16O2. Furthermore, the peak intensity ratio of S16O2 (Mz 64) to S16O18O (Mz 66) and S16O2 (Mz 64) to S18O2 (Mz 68) increased with increase in 16O2 concentration. This result suggests that 16O directly reacts with ZnS, if the concentration of 2 H218O is low. Therefore, the TPR was measured in the presence of very low concentration of H218O (0.4%) and compared with the result obtained in the absence of H218O. As shown in Figure 5, a small peak of Mz 66 was observed in the absence of H218O. This Mz 66 peak might be 34SO2 derived from a natural sulfur source that contained a small amount of 34S. The small amount of H218O drastically affected the distribution of the evolved sulfur dioxide: S16O18O (Mz 66) and a small amount of S18O2 (Mz 68) were evolved. The evolution of a large amount of S16O2 (Mz 64) suggests that the following reaction occurred:
ZnS + 3/2 16O2 w Zn16O + S16O2
(4)
Evolution of Sulfur Dioxides at a Constant Temperature. From Figure 5, the main reaction in the presence of the low concentration H218O was thought to be the reaction between 16O2 and ZnS. However, at low temperature (< 550 °C), it was expected that the main reaction was the reaction between H218O and ZnS. To confirm this hypothesis, the evolution of sulfur dioxide was measured under the same conditions as
Figure 6. Evolution of sulfur dioxide at constant temperature in the presence of a low concentration of H218O. Sample: 69% sulfided ZnO. Inlet gas: 3% 16O2-0.4% H218O-He. Reaction temperature: 525 °C.
those in Figure 5 at a constant temperature of 525 °C. As shown in Figure 6, the evolved sulfur oxides were S18O2 (Mz 68) and S16O18O (Mz 66), but S16O2 (Mz 64) evolution was observed only at the first stage of the oxidative regeneration of ZnS. From these results, it was confirmed that the reaction between H218O and ZnS predominately occurred at low temperatures. Mechanistic Consideration. From the experimental results, it was found that H2O played a very important role in the oxidative regeneration of ZnS. It was easily supposed that the H2O contributed to the reaction via an adsorbed state. Therefore the TPD of a H216O preadsorbed ZnS sample was measured. The sample ZnS was treated with a mixture of H216O (10%) and He for 0.5 h at 300 °C. As shown in Figure 7, two H216O evolution peaks were clearly observed. One of them was observed around at 150 °C and thought to be physically adsorbed H216O that might adsorb on the sample during cooling (the replacement of the system by the purge gas He was not perfect). The other peak (located on ca. 480 °C) was thought to be chemically adsorbed H216O on the ZnS. Therefore, it was expected that the H216O dissociated on the ZnS surface according to the following equation:
H216O S -16OH + -H
(5)
In the case of H218O, eq 5 may be expressed as follows:
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-Zn-S16O2 + -O16H (or -O18H) w S16O2 + -Zn16O (or Zn18O) + -H (20) -Zn-S16O18O + -O16H w S16O2 + -Zn18O + -H
Figure 7. H216O evolution from the preadsorbed ZnS sample during TPR. Sample: 69% sulfided ZnO. Preadsorption: 10% H216O-He, 300 °C for 0.5 h. 18
18
H2 O S - OH + -H
As it was suggested that the adsorbed dissociated over the ZnS, the reaction between ZnS and H218O was composed of the following elemental reaction route:
-ZnS + 18OH S -Zn-S18O + -H -Zn-S O +
18
18
18
-Zn-S O2 +
18
OH w -Zn-S O2 + -H 18
OH w -Zn-S O3 + -H
(7)
(9) (10)
2 (-H) w H2
(11)
-Zn, -H, -SO, -OH, and -O are surface species. The reaction of eq 1 is endothermic (∆H°298 ) 283 KJ/mol) and thermodynamically unfavorable (∆G°298 ) 265 KJ/ mol). Therefore, the reaction of eq 1 must be only one step of a oxidative regeneration route. The activated state of 16O2 in the oxidative regeneration of ZnS was unknown. However, in this discussion, we assumed that 16O2 adsorbed and dissociated over ZnS surface. The reaction of eq 3 may be expressed as follows:
O2 w 2 (-16O)
-H + -16O w -16OH 16
-ZnS + -16O w Zn-S16O
(22)
-Zn-S16O + -16O w -Zn-S16O2
(23)
-Zn-S16O2 + -16O w Zn16-S16O3
(24)
-Zn-S16O3 w S16O2 + Zn16O
(25)
(8)
-Zn-S18O3 w -ZnO + S18O2
16
In the absence of H218O or in the presence of a very low concentration of H218O, the reaction of eq 4 might contribute to the S16O2 formation. As the formation of S16O18O in the presence of H218O and S16O2 can be explained by the stepwise reactions, the reaction of eq 4 is thought to be composed of the following elemental steps in addition to eq 12:
(6) H216O
18
(21)
16
- OH + H S H2 O
(12) (13) (14)
The formation of S16O18O (Mz 66) could be explained by the following reactions:
-ZnS + -18OH (or -16OH) w -Zn-S18O (or -Zn-S16O) + -H (15) -Zn-S18O + -16OH w -Zn-S16O18O + -H
(16)
-Zn-S16O + -18OH w -Zn-S16O18O + -H
(17)
-Zn-S18O16O + -16OH (or -18OH) w 16 18 O O + -Zn16O (or -Zn18O) + -H (18) The formation of S16O2 (Mz 64) in the presence of H218O may be explained by the following reactions:
-Zn-S16O + -16OH w -Zn-S16O2 + -H
(19)
The confirmation of this mechanism needs much more study. Conclusions The role of H2O in the course of oxidative regeneration of zinc sulfide in the presence of O2 was studied using ZnO without support material as a sorbent. TPD technique and H218O were used to reveal the movement of the oxygen of H2O in the course of the oxidative regeneration of ZnS. From this study, the role of H2O was clarified: (1) ZnS was converted to SO2 and H2 by H2O in the absence of O2 at high temperature (> ca. 600 °C). (2) In the first step in the course of oxidative regeneration, ZnS predominantly reacts with H2O more than O2. (3) The SO2 formed from ZnS contained oxygen that came from H2O. The remaining problems in the oxidative regeneration of spent ZnO sorbents are the details of the role of O2 and the effect of the support material, which is usually added in practice. Literature Cited (1) Lew, S.; Jothimurugesan, K.; Flytzani-Stephanopoulos, M. High-Temperature H2S Removal from Fuel Gases by Regenerable Zinc Oxide Titanium Dioxide Sorbents. Ind. Eng. Chem. Res. 1989, 28, 535. (2) Westmoreland, P. R.; Harrison, D. P. Evaluation of Candidate Solids for High-Temperature Desulfurization of Low-Btu Gases. Environ. Sci. Technol. 1976, 10, 659. (3) Gangwal,S. K.; Harkins, S. M.; Woods, M. C.; Jain, S. C.; Bossart, S. J. Bench-Scale Testing of High-Temperature Desulfurization Sorbents. Environ. Prog. 1989, 8, 265. (4) Sa, L. N.; Focht, G. D.; Ranade, P. V.; Harrison, D. P. Modeling High-Temperature Desulfurization in a Fixed-Bed Reactor. Chem. Eng. Sci. 1989, 44, 215. (5) Focht, G. D.; Ranade, P. V.; Harrison, D. P. High-Temperature Desulfurization Using Zinc Ferrite: Regeneration Kinetics and Multicycle Testing. Chem. Eng. Sci. 1989, 12, 2919. (6) Jothimurugesan, K.; Harrison, D. P. Reaction between H2S and Zinc Oxide-Titanium Oxide Sorbent. 2. Single-pellet Sulfidation Modeling. Ind. Eng. Chem. Res. 1990, 29, 1167. (7) Woods, M. C.; Gangwal, S. K.; Jothimurugesan, K.; Harrison, D. P. Reaction between H2S and Zinc Oxide-Titanium Oxide Sorbent. 1. Single pellet Kinetic Studies. Ind. Eng. Chem. Res. 1990, 29, 1160.
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(8) Ayala, R. E.; Marsh, D. W. Characterization and LongRange Reactivity of Zinc Ferrite in High-Temperature Desulfurization Process. Ind. Eng. Chem. Res. 1991, 30, 55. (9) Silaban, J. T.; Harrison, D. P.; Berggren, M. H.; Jha, M. C. The Reactivity and Durability of Zinc Ferrite High-Temperature Desulfurization Sorbents. Chem. Eng. Commun. 1991, 107, 5571. (10) Lew, S.; Sarofim, A, F.; Flytzani-Stephanopoulos, M. Sulfidation of Zinc Titanate and Zinc Oxide Solids. Ind. Eng. Chem. Res. 1992, 31, 1890. (11) Sakurai, T.; Okarnoto, M.; Miyazaki, H.; Nakao, K. HighTemperature Desulfurization Performance and Durability of Fine Zinc Ferrite Particles Prepared by the Coprecipitation Method. Kagaku Kogaku Ronbunshu 1993, 20, 275. (12) Kobayashi, M.; Shirai, H.; Nunokawa, M. Investigation on Desulfurization Performance and Pore Structure of Sorbents Containig Zinc Ferrite. Energy Fuels 1997, 11, 887. (13) Sepelak, V.; Steinike, U.; Uecker, D.-C.; Trettin, R.; Wissmann, S; Becker, K. D. High-temperature Reactivity of Mechanosynthesized Zinc Ferrite. Solid State Ionics 1997, 101103, 1343. (14) Garcia, E.; Cilleruelo, C.; Ibrra, J. V.; Pineda, M.; Palacios, M. Thermogravimetric Study of Regenerable Sulfur sorbents for H2S Retention at High Temperature. Thermochim. Acta 1997, 306, 23 (15) Kobayashi, M.; Shirai, H.; Nunokawa, M. Elucidation of Sulfidation of Zinc Ferrite in a Reductive Gas Environment by in Situ X-ray Diffraction Analysis and Mossbauer Spectroscopy. Ind. Eng. Chem. Res. 2000, 39, 1934.
(16) Sasaoka, E.; Hirano, S.; Kasaoka, S.; Sakata, Y. Stability of Zinc Oxide High-Temperature Desulfurization Sorbent For Reduction. Energy Fuels 1994, 8, 763-769. (17) Sasaoka, E.; Hirano, S.; Kasaoka, S.; Sakata, Y. Characterization of Reaction between Zinc Oxide and Hydrogen Sulfide. Energy Fuels 1994, 8, 1100-1105. (18) Sasaoka, E.; Taniguchi, K.; Hirano, S.; Uddin, M. A.; Kasaoka, S.; Sakata, Y. Characterization of Reaction between ZnO and COS. Ind. Eng. Chem. Res. 1996, 35, 2389-2394.. (19) Sasaoka, E.; Taniguchi, K.; Hirano, S.; Uddin, M. A.; Kasaoka, S.; Sakata, Y. Catalytic Activity of ZnS Formed from Desulfurization Sorbent ZnO for Conversion of COS to H2S. Ind. Eng. Chem. Res. 1995, 34, 1102-1106. (20) Sasaoka, E.; Iwamoto Y.; Hirano, S.; Uddin, M. A.; Sakata, Y. Soot Formation over Zinc Ferrite High-Temperature Desulfurization Sorbent. Energy Fuels 1995, 9, 344-353. (21) Sasaoka, E.; Sada, N.; Manabe, A.; Uddin, M. A.; Sakata, Y. Modification of ZnO-TiO2 High-Temperature Desulfurization Sorbent by ZrO2 addition. Ind. Eng. Chem. Res 1999, 38, 958963.
Received for review May 11, 2000 Revised manuscript received July 6, 2000 Accepted July 10, 2000 IE0004747
